Field of the Invention
The present invention relates generally to filters for particulates, and more particularly to a new fabric for use in filter media which is capable of use in high heat environments having a high airflow requirement and a method for making this new high heat filter fabric.
As industrialized countries increasingly seek to limit air pollution, the standards for emissions have become increasingly stringent. At the same time, with the demand for electricity continuing to rise and the limited use of emission-free nuclear power plants, the use of fossil fuels including coal has continued to increase. Coal-fueled power plants present a particular problem, with the particulates being contained in emissions which are at a relatively high temperature.
While such emissions have in the past generally been treated with electrostatic precipitators, the increasingly stringent emission standards are driving a replacement of the precipitators with fabric filters. There are several factors in the new air quality standards which are increasing the desirability of fabric filters. The first of these is the new ambient standards for fine particulates (particulates which are 2.5 microns in diameter and smaller). Recent U.S. Environmental Protection Agency regulations will require power plants to reduce such emissions in the years from 2008 to 2013.
Pulse-jet filters use nonwoven fabrics and membranes which are capable of withstanding temperatures of 400 degrees Fahrenheit. There is a strong potential for filter media which is capable of use in even higher ambient temperatures, from 450 degrees Fahrenheit to approximately 850 degrees Fahrenheit. At such temperatures, the filter media can be used in the exhaust air flow ahead of a catalytic converter, which is used to reduce NOx. This would be highly advantageous since catalytic converters are subject to plugging, and the elimination of particulates, including fine particulates, upstream of the catalytic converter would increase catalytic converter life and decrease capital costs.
Another requirement is that power plants further reduce the levels of SO2 emissions, with this requirement being even more stringent with regard to regulations relating to power plants which are located near to national parks. Reduction of these emissions is best accomplished through the use of dry scrubber and fabric filter emission abatement systems. One technique which is used involves the injection of lime into the hot gas stream, which results in gypsum powder which must be removed from the hot gas stream. This is best accomplished through the use of fabric filters in pulse-jet filters.
Two other regulatory factors also indicate the use of fabric filters in power plant emissions control. The first factor is the increasingly stringent regulatory requirements for the elimination of mercury emissions, which is accomplished by injecting activated carbon particles into the hot gas stream and then using a fabric filter to remove the mercury which is entrained by the carbon particles. The second is regulatory requirements to limit the emission of toxic metals, including lead, cadmium, arsenic, chromium, and other toxic metals. These regulations require the use of the best available control technology (BACT), which at the present time is the use of fabric filters which remove toxic metals in the course of removing fine particulates from the hot gas stream.
Other industries are also turning increasingly to the use of fabric filters to treat hot gas streams and remove fine particulates from such hot gas streams. For example, the steel industry is also experiencing a significant increase in the use of fabric filters to remove fine particulates from hot gas streams. The waste incineration industry, which is increasing as the use of landfills is decreased, is also being required to remove acid gases, mercury, dioxins, and dust. The use of both lime injection and activated carbon injection together with fabric filters in the hot gas stream is viewed as the best manner in which to filter the exhaust gases.
While it will thus be appreciated that there exists a tremendous need for fabric filter media which can operate in a high temperature gas stream, there also exists a strong need for a fabric filter media in other applications which is capable of efficiently removing fine particulates from a gas stream. This includes a wide variety of industries and fine particulate generators ranging from shot blast equipment to textile manufacturing plants, with gas streams to be treated ranging in temperature from ambient temperature to relatively high temperatures (approximately 300 to 350 degrees Fahrenheit). All of these industries also represent potential markets for an efficient filter fabric capable of removing fine particulates and operating at a wide range of temperatures.
While the prior art has sought fabrics which are nonflammable, and includes a number of different approaches to producing fabrics which are nonflammable, there is strong unremedied need in the filter fabric area for a filter fabric which is capable of operating at a high ambient temperature. While most of the prior art dealt with the treatment of fabrics to make them fire-resistant, a few prior art references have dealt with the manufacture of a fabric which is inherently fire-resistant.
One example of such an approach is found in U.S. Pat. No. 4,513,042, to Lumb, which is hereby incorporated herein by reference. Lumb discloses a knit fabric which is made using a flame retardant aramid yarn with a pile made of flame retardant rayon fibers and superwashed wool fibers. The Lumb fabric is used as for the manufacture of fire-resistant cold weather gear for firefighter and military applications. However, the Lumb fabric is not suitable for use as a filter fabric, and is designed for transient protection against flame (for a matter of 16 to 37 seconds as disclosed in Lumb), and is simply not suitable for operation in a high ambient temperature environment where temperatures are typically in excess of 400 degrees Fahrenheit.
It is accordingly the primary objective of the present invention that it provide a filter fabric material which is suitable for use in dust and/or particulate filters that will be used in high temperature ambient operating environments. It is a closely related objective of the high heat filter fabric of the present invention that it be highly efficient as a filter medium at the removal of fine particulates which are as small as or possibly even smaller than 2.5 microns. It is another related objective of the high heat filter fabric of the present invention that it be capable of continuous operation in an environment wherein the ambient temperature is typically in excess of 400 degrees Fahrenheit.
In is an additional objective of the high heat filter fabric of the present invention that it also be capable of operating in high temperature environments without experiencing any significant degradation of the filter fabric material. It is a further objective of the high heat filter fabric of the present invention that it be reusable after periodic cleaning to remove particulates which have been trapped by the filter medium. It is yet another objective of the high heat filter fabric of the present invention that it work as a filter medium with any of a wide variety of different high temperature air pollution abatement technologies, including (but not limited to) both lime injection and activated carbon injection. It is still another objective of the high heat filter fabric of the present invention that it provide a sufficiently high degree of airflow therethrough to function properly in high airflow environments.
The high heat filter fabric of the present invention must also be both durable and long lasting, so that filter bags made from the fabric of the present invention will require little or no maintenance to be provided by the user throughout their operating lifetime. In order to enhance the market appeal of the high heat filter fabric of the present invention, it should also be of relatively inexpensive construction to thereby afford it the broadest possible market. Finally, it is also an objective that all of the aforesaid advantages and objectives of the high heat filter fabric of the present invention be achieved without incurring any substantial relative disadvantage.